It's an indicator of moisture in the oil.
Do you take mostly short trips in the car on a regular basis? If the car dosen't reach normal operating temperature, you can get this condition. Also, if your engine is running too cool (removed thermostat for example ) it's also a possibility. Aeration of the oil is another.
Lastly, it's a possible sign of headgasket problems. If none of the above apply, you might want to get a pressure test and see what's up. Good luck Joe.